1. Field of the Invention
The present invention relates to an active-matrix display device including light-emitting elements in its pixels. More specifically, the invention relates to a technique for improving the reliability of thin film transistors formed in the pixels.
2. Description of the Related Art
In recent years, development of flat self-luminous display devices including organic EL devices as light-emitting elements is being actively promoted. The organic EL device is based on a phenomenon that an organic thin film emits light in response to application of an electric field thereto. The organic EL device can be driven by application voltage of 10 V or lower, and thus has low power consumption. Furthermore, because the organic EL device is a self-luminous element that emits light by itself, it does not need an illuminating unit and thus easily allows reduction in the weight and thickness of the display device. Moreover, the response speed of the organic EL device is as very high as about several microseconds, which causes no image lag in displaying of moving images.
Among the flat self-luminous display devices including the organic EL devices in the pixels, particularly an active-matrix display device in which thin film transistors are integrally formed as drive elements in the respective pixels is being actively developed. The active-matrix flat self-luminous display device is disclosed in e.g. Japanese Patent Laid-open No. 2007-310311.
FIG. 23 is a schematic circuit diagram showing one example of the active-matrix display device of a related art. The display device includes a pixel array part 1 and a peripheral drive part. The drive part includes a horizontal selector 3 and a write scanner 4. The pixel array part 1 includes signal lines SL disposed along the columns and scan lines WS disposed along the rows. Pixels 2 are disposed at the intersections of the signal lines SL and the scan lines WS. FIG. 23 shows only one pixel 2 for easy understanding. The write scanner 4 includes shift registers. The shift registers operate in response to a clock signal ck supplied from the external and sequentially transfer a start pulse sp supplied from the external similarly, to thereby sequentially output a control signal to the scan lines WS. The horizontal selector 3 supplies a video signal to the signal lines SL in matching with the line-sequential scanning by the write scanner 4.
The pixel 2 includes a sampling transistor T1, a drive transistor T2, a hold capacitor C1, and a light-emitting element EL. The drive transistor T2 is a P-channel transistor. The source thereof is connected to a power supply line and the drain thereof is connected to the light-emitting element EL. The gate of the drive transistor T2 is connected to the signal line SL via the sampling transistor T1. The sampling transistor T1 is turned on in response to the control signal supplied from the write scanner 4 to thereby sample the video signal supplied from the signal line SL and write it to the hold capacitor C1. The drive transistor T2 receives, at its gate, the video signal written to the hold capacitor C1 as a gate voltage Vgs, and causes a drain current Ids to flow to the light-emitting element EL. This causes the light-emitting element EL to emit light with the luminance dependent on the video signal. The gate voltage Vgs refers to the potential of the gate relative to that of the source.
The drive transistor T2 operates in the saturation region, and the relationship between the gate voltage Vgs and the drain current Ids is represented by the following characteristic equation.Ids=(½)μ(W/L)Cox(Vgs−Vth)2 In this equation, μ denotes the mobility of the drive transistor, W denotes the channel width of the drive transistor, L denotes the channel length of the drive transistor, Cox denotes the capacitance of the gate insulating film of the drive transistor per unit area, and Vth denotes the threshold voltage of the drive transistor. As is apparent from this characteristic equation, when operating in the saturation region, the drive transistor T2 functions as a constant current source that supplies the drain current Ids depending on the gate voltage Vgs.
FIG. 24 is a graph showing the voltage-current characteristic of the light-emitting element EL. In this graph, an anode voltage V is plotted on the abscissa and the drive current Ids is plotted on the ordinate. The anode voltage of the light-emitting element EL is equivalent to the drain voltage of the drive transistor T2. The light-emitting element EL has a tendency that its current-voltage characteristic changes over time and the characteristic curve gradually falls down with time elapse. Therefore, the anode voltage (drain voltage) V changes even if the drive current Ids is constant. However, in the pixel circuit 2 shown in FIG. 23, the drive transistor T2 operates in the saturation region and allows the flowing of the drive current Ids dependent on the gate voltage Vgs irrespective of change in the drain voltage. This makes it possible to keep the light-emission luminance constant irrespective of aging change in the characteristic of the light-emitting element EL.
FIG. 25 is a circuit diagram showing another example of a related-art pixel circuit. This pixel circuit is different from the pixel circuit shown in FIG. 23 in that the drive transistor T2 is not a P-channel transistor but an N-channel transistor. In many cases, it is more advantageous that all of the transistors included in the pixel are N-channel transistors in terms of the circuit manufacturing process.